How Our Warming Climate May Affect Our Fishery

By Bruce Davidson

In early November of 2021 at the climate talks in Glasgow, the world was presented with some convincing evidence that global warming was very real and expected to persist.  Whether or not one agrees with the premise that humans are responsible for the rising temperatures (something that is beyond question to most attendees) we Carling folk just experienced the warmest Georgian Bay waters on record this fall.  As a lifelong cottager on the Bay, I personally opened at the earliest date ever, closed at the latest date ever, and swam (arguably in the view of my wife) at the latest date ever. Get used to it.  This warming climate is going to have consequences for our fishery, so I did a little research to see what they might be.

In order to better understand the subject, it helps to appreciate what happens to the waters in our beloved Bay over the course of a year.   Under ice cover, the water temperatures are pretty much the same from the surface right down to the bottom.  These waters can circulate freely, which provides uniform fish habitat.  Once the ice melts a thin layer of oxygen-rich water develops at the surface that draws fish into the shoreline shallows.  Because water has the interesting property of reaching maximum density at +4C, a remarkable phenomenon known as the spring turnover occurs in early spring when surface water reaches this temperature. Surface water, now at maximum density sinks and, aided by mixing from winds, creates a uniform column of water at the same density and temperature. Fish can be anywhere at this time.  Things start to get really interesting again in early summer.  Water starts to segregate into three distinct layers.  The warm oxygenated surface waters now well above +4C are light and strongly resist mixing.  Beneath these waters now lies the thermocline, a layer where temperatures decrease rapidly with depth (as you will quickly discover when you dive down to retrieve the anchor which you stupidly tossed overboard before tying it to the boat).  Below the thermocline is the hypolimnion, a layer of deep, cold water that remains relatively undisturbed but now has limited oxygen replenishment.  To complete the picture of a dynamic system, later in the year, things go into reverse.  Firstly, as the surface waters cool, they increase in density, sink, and start to erode the thermocline.  Eventually, surface waters get back to +4C when we experience the fall turnover with mixing occurring throughout the water column at uniform temperature and density.  Finally, when the surface waters go below +4C they become lighter once again and float on the warmer deeper water before ultimately freezing.

Thermal stratification in a lake is important because it affects the distribution of temperature-sensitive fish. When things get really warm cold-water species not only have to be able to find cooler temperatures in deep water but there has to be enough oxygen there as well.  As we have learned above, in late summer there is not much mixing in the waters and the hypolimnion may have lost much of its oxygen.  So cold water fish may face the tragic dilemma of death by temperature or death by oxygen starvation. 

Bottom line:  As our climate warms, cold-water fish such as lake trout and brook trout and whitefish will decline, particularly in southern Ontario.  Cool water fish such as pike, muskellunge, walleye and perch will fare somewhat better for obvious reasons and may also be able to move farther north to extend their range.  The good news is that warm water species such as largemouth bass are projected to benefit from these changes with both increased habitat and a more productive ecosystem.  Smallmouth bass like somewhat cooler water but will likely flourish with increased availability of food as well.  So, any Carling fishermen expecting to be pursuing their passion in the decades to come might want to consider trading in their downriggers for bass boats with spinning gear.  I will, of course, be watching from my chaise lounge partially immersed in the nice warm waters of the more temperate Georgian Bay.